The general objective of this program project is to examine behavioral, anatomical and biochemical changes that occur during normal aging of the nervous system of mammals, and the potential of the aging nervous system to compensate for these changes. To accomplish these goals, a number of systems are being examined. In the auditory system the effect of age on the ability of rats to acquire and maintain auditory discrimination are being studied, and this ability is being correlated with morphological age changes in the cochleas, cochlear nuclei and spiral ganglion. The morphological changes include examination of the loss of receptor cells, changes in synapses between neurons, and accumulation of pigment. More centrally, the ability of neurons in the auditory cortex to respond to deafferentation at varying ages are being examined to determine if loss of the callosal input results in compensatory growth of afferents entering the cortex from the thalamus. In the olfactory system changes in the receptor cells, the morphology of neurons and their synaptic input are being studied as rats age. These studies will also indicate if the aging nervous system still exhibits plasticity, and collateral sprouting of axons is being further studied in the dentate gyrus. A possible important factor in the aging of neurons is the state of the microvascular network, and this is being examined both biochemically and anatomically. Additional biochemical studies are being carried out on nuclear chromatin, with which the effects of age are being assessed by determining the susceptibility of chromatin to cleavage by bacterial nuclease.